Terephthalic acid and other aromatic carboxylic acids are widely used in the manufacture of polyesters, commonly by reaction with ethylene glycol, higher alkylene glycols or combinations thereof, for conversion to fiber, film, containers, bottles and other packaging materials, and molded articles.
In commercial practice, aromatic carboxylic acids are commonly made by liquid-phase oxidation in an aqueous acetic acid solvent of methyl-substituted benzene and naphthalene feedstocks, in which the positions of the methyl substituents correspond to the positions of carboxyl groups in the desired aromatic carboxylic acid product, with air or another source of oxygen, which is normally gaseous, in the presence of a bromine-promoted catalyst comprising cobalt and manganese ions. The oxidation is exothermic and yields aromatic carboxylic acid together with high- and low-molecular weight byproducts, including partial or intermediate oxidation products of the aromatic feedstock, and acetic acid decomposition reaction products, such as methanol, methyl acetate, and methyl bromide. Water is also generated as a byproduct. Aromatic carboxylic acids, typically accompanied by oxidation byproducts of the feedstock, are commonly formed dissolved or as suspended solids in the liquid-phase reaction mixture and are commonly recovered by crystallization and solid-liquid separation techniques.
The exothermic oxidation reaction is commonly conducted in a suitable reaction vessel at elevated temperature and pressure. A liquid-phase reaction mixture is maintained in the vessel and a vapor phase formed as a result of the exothermic oxidation is evaporated from the liquid phase and removed from the reactor to control reaction temperature. The vapor phase comprises water vapor, vaporized acetic acid reaction solvent and small amounts of byproducts of the oxidation, including both solvent and feedstock byproducts. It usually also contains oxygen gas not consumed in the oxidation, minor amounts of unreacted feedstock, carbon oxides and, when the oxygen source for the process is air or another oxygen-containing gaseous mixture, nitrogen and other inert gaseous components of the source gas.
The high temperature and pressure vapor phase generated by the liquid-phase oxidation is a potentially valuable source of recoverable acetic acid reaction solvent, unreacted feed material and reaction byproducts, as well as energy. However, its substantial water content, high temperature and pressure and corrosive nature due to components such as gaseous methyl bromide, acetic acid solvent and water pose technical and economic challenges to separating or recovering components for recycle and recovering its energy content. Further, impurities that remain unseparated in recovered process streams can prevent re-use of streams if impurities adversely affect other process aspects or product quality.
Purified forms of aromatic carboxylic acids are usually favored for the manufacture of polyesters for important applications, such as fibers and bottles, because impurities, such as the byproducts generated from the aromatic feedstocks during oxidation and, more generally, various carbonyl-substituted aromatic species, are known to cause or correlate with color formation in polyesters made from the acids and, in turn, off-color in polyester converted products.
Preferred purified forms of terephthalic acid and other aromatic carboxylic acids with lower impurities contents, such as purified terephthalic acid or “PTA”, are made by catalytically hydrogenating less pure forms of the acids, such as crude product comprising aromatic carboxylic acid and byproducts generated by the liquid-phase oxidation of the aromatic feedstock or so-called medium purity products, in solution at elevated temperature and pressure using a noble metal catalyst. Purification not only removes impurities from the crude and medium purity products, particularly the major impurity, 4-carboxybenzaldehyde, but also reduces the level of color bodies and the amount of metals, acetic acid and bromine compounds. In commercial practice, liquid-phase oxidation of alkyl aromatic feed materials to crude aromatic carboxylic acid and purification of the crude product are often conducted in continuous integrated processes in which crude product from liquid-phase oxidation is used as the starting material for purification.
Reducing or eliminating the production of impurities, color bodies and carbon oxides from such commercial processes continues to be an ongoing challenge. One solution may be found in an alternative process for the manufacture of aromatic carboxylic acids from feedstocks other than methyl-substituted benzene and naphthalene feed materials.
The U.S. Department of Energy (“DOE”) has recently identified 12 top-tier chemical building blocks from biomass processing, as reported in the Biomass Report for the DOE Office of Energy Efficiency and Renewable Energy entitled Top Value Added Chemicals from Biomass, Volume 1—Results of Screening for Potential Candidates from Sugars and Synthesis Gas, August 2004. Among the twelve building blocks identified by the DOE is 2,5-furandicarboxylic acid. The DOE has been soliciting proposals for the use of 2,5-furandicarboxylic acid in the production of commodity chemicals, such as polyesters.
It is generally known that biomass carbohydrates can be enzymatically converted to fructose and other sugars. Under facile dehydration conditions, these sugars are then converted to 5-hydroxymethylfurfural, which is readily oxidized to 2,5-furandicarboxylic acid. It has been reported that of the approximately 200 billion tons of biomass produced per year, 95% of it is in the form of carbohydrates, and only 3 to 4% of the total carbohydrates are currently being used for food and other purposes. Thus, there is an abundant untapped supply of biomass carbohydrates, which can potentially be used for the production of non-petroleum based commodity chemicals that are fully renewable.
Accordingly, it would be desirable to provide a process for the production of terephthalic acid from a feedstock other than a conventional alkyl aromatic feed material, such as paraxylene, which not only reduces or eliminates the production of impurities, color bodies and carbon oxides, but also eliminates the need for the purification step in current commercial processes. It would also be desirable if the alternative feedstock utilized in the process was derived from biomass.